1. Field of the Invention
This invention relates to an optical semiconductor apparatus in which an optical semiconductor device and an electrical circuit device are incorporated in a single body.
2. Description of the Prior Art
With significant advancements in such fields as optical information processing, light communication, etc., it has become necessary to miniaturize optical modules and improve their capabilities by the use of optoelectronic integrated circuits (OEIC), in which optical semiconductor devices such as semiconductor lasers, photodetectors, etc., and electrical circuit devices such as operating circuits, modulators, etc., are integrated onto the same substrate. In a conventional OEIC, since a number of electrical circuit devices are formed on the same substrate, they must be electrically insulated from each other. Moreover, since a number of optical semiconductor devices having different structures are formed on the same substrate, the use of a step-shaped substrate is unavoidable, causing difficulties in growing the required crystals. Especially, in cases where semiconductor lasers are used as optical semiconductor devices, the semiconductor devices must be placed at a great distance from the heat-radiating board on which the OEIC should be mounted, since they must be integrated onto a semi-insulating substrate which is thick enough to also support electrical circuit devices thereon, causing a decrease in the heat-radiation effect of the semiconductor lasers. Semi-insulating substrates which can be used to support and insulate semiconductor lasers and electrical circuit devices have a high defect density which causes a decrease in the reliability of the semiconductor lasers formed thereon or an increase in the dark current in photodetectors which can be used instead of the semiconductor lasers.
FIG. 2 shows a conventional OEIC, wherein a semiconductor laser and a field effect transistor for operating the semiconductor laser, respectively, are formed on the upper portion and the lower portion of a GaAs semi-insulating step-shaped substrate 1 and both devices are electrically connected to each other by an n-GaAs wiring layer 2. The OEIC is produced as follows: After growth of an active layer 9 for the field effect transistor, the area other than the portion of the substrate 1 corresponding to the semiconductor laser is coated with a SiO.sub.2 film 3, followed by the selective growth of an n-GaAlAs cladding layer 4, a GaAlAs active layer 5, a p-GaAlAs cladding layer 6 and a p-GaAs cap layer 7, resulting in a doubleheterostructure for laser oscillation. Then, the SiO.sub.2 film on the field effect transistor area is removed, and a source electrode 10, a gate electrode 11 and a drain electrode 12, all of which serve as electrodes for the field effect transistor, are formed on the active layer 9. A p-side electrode 13 for laser oscillation is formed on the cap layer 7. The resulting OEIC is mounted on a heat-radiating board through the substrate 1 having the thickness of as much as approximately 100 .mu.m, which causes the suppression of radiation of heat which generates in the laser oscillation operation. Moreover, the height of the step of the substrate, although depending upon the precision of the etching processing for the step, is as much as approximately several .mu.m, which causes difficulty in the succeeding photolithographic process for the formation of the doubleheterostructure. The use of the semi-insulating substrate also causes a decrease in the reliability of the semiconductor laser.